To address and overcome inherent existing limitations in prior art radio frequency identification (RFID) tags with respect to cost, data capacity and reliable range, a new technology utilizing SAW devices as identification tags has been developed. SAW tags are described in detail in U.S. patent application Ser. No. 10/024,624, entitled “Surface Acoustic Wave Identification Tag Having Enhanced Data Content and Methods of Operation and Manufacture Thereof,” Hartmann, Clinton S., commonly assigned with the invention and incorporated herein by reference. The principles used to encode data on SAW tags involving simultaneous phase and time shift modulation are described in detail in U.S. patent application Ser. No. 10/062,833, entitled “Modulation by Phase and Time Shift Keying and Method of Using the Same,” Hartmann, Clinton S., also commonly assigned with the invention and incorporated herein by this reference. The principles used to encode data by combining multi-pulse per group modulation with simultaneous phase and time shift modulation are described in detail in U.S. patent application Ser. No. 10/062,894, entitled “Modulation by Combined Multi-pulse per Group with Simultaneous Phase and Time Shift Keying and Method of Using the Same,” Hartmann, Clinton S., also commonly assigned with the invention and incorporated herein by reference. Additional pertinent information regarding SAW identification tags and SAW identification tag readers is set forth in detail in U.S. Pat. No. 6,708,881B1, entitled “Reader for a High Information Capacity Saw Identification Tag and Method of Use Thereof,” Hartmann, Clinton S., again commonly assigned with the invention and incorporated herein by reference.
An interrogated RFID tag reflects or retransmits a radio signal in response to an interrogation signal. The returned or reply signal contains data that, when decoded, identifies the tag and any object with which the tag is associated. A SAW device used as an identification tag can be encoded with a large amount of data. When encoded with 64 or 96 bits of data, in accordance with certain electronic product code (EPC) specifications, a reliable system and procedure to accurately identify the tag from a distance is required. Frequently other electronic devices will also be in use in the environment where RFID tags are used. The signals transmitted by these other devices adds to the difficulty in detecting responses to an interrogation pulse.
The problem can be best understood in the context of a user that has a large number of objects, each with its own unique identification tag. Added to the fact that a large number of identification tags are returning signals in response to an interrogation pulse, there most probably are other radio frequency signals present. For example, a SAW identification tag system used in a warehouse or shipping facility will most likely be operated in an environment where a wireless local area network (LAN) is also in operation. To identify a specific object among a large number of objects, an interrogation signal will be transmitted that will simultaneously generate a response from each SAW tag on each object. Not only must the SAW tag reader be able to identify the signals being returned from the SAW identification tags, it must also assure that its interrogation signal and the responses to such signal do not interfere with the wireless LAN. In addition, the SAW tag reader must also be able to cope with any signal interference caused by the wireless LAN. Thus, it is important for SAW tags to be encoded in a manner that permits tags to be readily distinguished from each other. It is equally as important that the SAW tag reader be able to discriminate SAW tag responses from other electronic signals and that signals from the SAW identification tag system not interfere with other devices. Methods are needed to encode and read SAW tags so that the unique data on the SAW tags can readily be distinguished. Methods are also needed to permit a SAW identification tag system to operate in an environment where other signals are present. Methods are also needed that permit the operation of SAW identification tag systems in a manner that does not interfere with other devices.
Accordingly, what is needed in the art are methods to operate and use a SAW identification tag system in an environment with other signal generating electronic devices and still reliably discriminate between multiple SAW tag responses.